Fisiología Endocrina - INTRO (CONTROL, TRANSPORTE, ACLARAMIENTO) PARTE 2/3 (IG:@doctor.paiva)

Introduction to Endocrinology: Hormonal Control, Transport, and Clearance

Overview of Hormonal Action

• The initiation and duration of hormone secretion vary significantly among different hormones, influenced by their specific control functions. For instance, adrenaline is secreted within seconds and acts quickly, while growth hormone and thyroxine take months to exert their full effects.

• Hormone concentrations in the blood are typically very low, ranging from picograms to micrograms. Daily secretion rates are also minimal, measured in milligrams or micrograms.

Mechanisms of Hormonal Control

• Hormonal secretion is regulated by various mechanisms that ensure effective control. Two primary types include:

• Nervous Control: Triggered by nervous stimulation (e.g., catecholamines released from the adrenal medulla upon sympathetic nerve activation).

• Feedback Control: This mechanism is more significant than nervous control and includes:

• Negative Feedback: Counteracts initial stimuli; for example, increased substance concentration inhibits further secretion.

• Positive Feedback: Amplifies the initial stimulus; less common but crucial in certain processes.

Examples of Feedback Mechanisms

• Negative feedback examples include:

• Increased blood glucose stimulates insulin release from the pancreas, lowering glucose levels.

• Conversely, decreased glucose triggers glucagon release to raise blood sugar through glycogenolysis.

• Another example involves the hypothalamic-pituitary-thyroid axis where:

• TRH (Thyrotropin-Releasing Hormone) stimulates TSH (Thyroid-Stimulating Hormone), which promotes thyroid hormone production. Elevated thyroid hormones inhibit TRH and TSH release.

Positive Feedback Mechanism

• An example of positive feedback is seen with Luteinizing Hormone (LH):

• Estrogen increases LH production before ovulation, leading to higher estrogen synthesis and further LH secretion until a peak concentration is reached.

Understanding Hormonal Feedback Mechanisms

Positive Feedback in Coagulation

• The transformation of protein into thrombin initiates a positive feedback loop, where activated factor 5 further converts prothrombin into more thrombin, enhancing clot formation and preventing blood loss after hemorrhage.

• Thrombin also converts fibrinogen to fibrin, solidifying the blood clot. This exemplifies positive feedback, contrasting with negative feedback which is more commonly utilized in biological systems.

Cyclical Variations in Hormone Release

• Hormonal release exhibits cyclical variations influenced by factors such as seasonal changes, developmental stages, aging processes, diurnal cycles, and sleep patterns.

• Circadian rhythms significantly affect hormone levels; for instance, cortisol peaks around 8 AM and drops between midnight and 4 AM.

Hormones and Nutritional Influence

• Insulin secretion varies based on food intake, increasing post-meal to facilitate glucose uptake.

Types of Hormones Based on Chemical Structure

• There are three main types of hormones: peptides (hydrosoluble), catecholamines (including thyroid hormones), and steroids (liposoluble).

• Peptide hormones circulate freely in plasma while thyroid hormones and steroids bind to specific plasma proteins; over 99% of thyroxine is protein-bound.

Biological Activity of Hormones

• Only free-circulating hormones can exert biological effects; those bound to plasma proteins lack activity until dissociated.

• The clearance rate of hormones from the bloodstream depends on their secretion rhythm and metabolic clearance rate—measured as the volume of plasma cleared per minute.

Elimination Mechanisms for Hormones

• Hormones are eliminated through various mechanisms including metabolic destruction by tissues or excretion via liver bile or kidneys.

• Peptide hormones typically degrade quickly due to enzymatic action in blood/tissues; for example, angiotensin II has a half-life under one minute.

Half-Life Differences Among Hormones

• Steroid hormones have longer half-lives ranging from 20 to 100 minutes while thyroid hormones can last from one day up to six days when bound to proteins.